Bolt Torque Calculator

Because friction varies in the real world, surface condition, coating thickness and lubricant batches all shift it. A single value is false precision; a range is the honest engineering answer.

With better lubrication, less torque is lost to friction and more converts into preload. Reaching the same preload therefore requires significantly less torque.

Beyond proof load the bolt enters plastic deformation, clamp force no longer grows linearly with torque, and permanent elongation or fracture may follow. Use the torque check above to confirm your margin; run a VDI 2230 analysis for critical joints.

Fine threads have a larger stress area As (e.g. M14 fine = 125 mm² vs 115 mm² coarse), so preload and torque are higher at the same class; but fine threads gall more easily and demand cleaner assembly. The torque difference roughly follows the As ratio (≈ +9% for M14).

K can range from 0.10 to 0.30 depending on coating, lubrication and washer stack, even batches from one supplier drift. Preset ranges are a first estimate; the accurate value must be measured on the production-identical stack (ISO 16047 method).

Austenitic A2/A4 stainless is tough and work-hardens rapidly; frictional heating can locally weld thread flanks. Use anti-seize and follow the Greased/MoS₂ or Waxed/PTFE presets (K ≈ 0.10–0.12).

1 lbf·ft ≈ 1.356 N·m (built into this tool). But SAE grades and ISO 8.8/10.9 are different strength systems, never convert the unit without re-checking the grade.

Anti-seize drops K into the 0.10–0.12 band. Following a dry-state torque table after applying it overloads the joint, and vice versa. Always match the preset to the actual lubrication state.

Fatigue-critical joints under cyclic loading, non-steel clamped parts, very large sizes, or applications with certified preload accuracy requirements.

By torque control: T = K · Pc · d, where Pc is the specified preload and d the nominal diameter. Take Pc from the design documents (GB/T 1231-2024 Table 14 lists the preload range for 10.9S and 8.8S, and site practice often uses its mid value), and take K from the torque coefficient measured on the delivered batch — never carry over a value from another batch. The acceptance band is 0.110–0.150, and a sample of eight assemblies is normally calibrated. Site practice: the initial pass is about 50% of the final torque, with final tightening on the same day.

Because tension control assemblies (GB/T 3632) are not tightened by torque control at all: the wrench grips the splined end and shears it off once the specified preload is reached, so the preload is set by the bolt and the calibrated wrench rather than by a torque value. Publishing a torque table would invite the wrong tightening method. For weights and sizes of tension control assemblies, use the GB/T 3632 section of the Bolt Weight Calculator.

For a 12.9 socket head cap screw, T = K · d · F with K = 0.20 (dry) or 0.13 (oiled) and F = 70% of Rp0.2 × As. For M10 × 12.9 that is roughly 58–89 N·m and for M6 × 12.9 about 12–19 N·m, depending on lubrication. Treat the table as an estimate and confirm it against your own coating and lubrication stack.